Composite materials formed with long fibers embedded in a rigid organic matrix are well known and widely implemented in the production of structure parts, whether being heavy loaded working structures or simple fairings more slightly loaded.
The advantages of such composite material parts are various but, according to the intrinsic performances of fibers, the used resins and to the applied implementation techniques, the parts are obtained with possible benefits on the resistance, the mass, the complexity or the cost, in particular, relatively to equivalent parts which would be made from metallic materials.
One more generally finds parts which are produced by implementing glass fibers, aramid fibers or carbon fibers or a combination of these fibers.
Apart from parts of limited dimensions and resistances, or from particular applications like those implementing a three dimensions said 3D weaving, the fibers are generally arranged in successive layers within the thickness of the parts to be produced in order to form plies whose the accumulation forms the local thickness of the part and gives to the latter its resistance and its rigidity when the fibers are embedded into the hard matrix.
A characteristic of the considered composite parts is their low intrinsic electric conduction because of the implementation of matrices containing insulating organic resins and of the fibers which are themselves made from materials included into the category of electrical insulators (aramid fibers or glass fibers) or into the category of materials considered as slightly conductive or as bad electrical insulators, typically the carbon fibers in the considered application field.
A similar behavior is also observed, on the thermal level, with respect to these materials for which the mechanical performances have generally a priority on the thermal conduction characteristics at the choice time of the materials to implement in order to produce a part.
In some applications, in particular when currents must be transported by the structure, it is necessary to deposit on the surface of the composite material structures, electrically conductive elements.
Thus in the case of the aircraft structures which are likely to be subjected to lightning strikes, a metal netting, generally made of bronze or copper, is fixed on a face of the composite material parts, a netting of which the surface density is adapted to the currents having to be supported to disperse the currents induced by the lightning strikes.
In the absence of such a metallization the composite material structure, unable to disperse the energy brought by an lightning strike, is likely to be damaged locally by the lightning and to require in this case urgent repairs.
Moreover, contrarily to the metallic structures, these composite structures are not able to provide the function of current return for an electric system and it is, for example necessary in the case of aircraft with a composite material structures, to install electric conductors in order to ensure the current returns for the electric systems or to use elements having a metallic structure and presenting an electrical continuity adapted with a sufficient reliability to ensure the current return for the electric systems.
To such a solution, constraining in itself at the design level by the respect of the required electric routes and electrical continuity necessary and by the protection constraints of the metallic structures relatively to the composite structures when those use carbon fibers, in particular, is added to the need of metallization of the structure in order to protect it from the effects of the lightning and it proves to limit the expected benefit of the generalized use of composite material structures, in particular for aircraft.
When it is necessary to increase the thermal conductivity of the part structure, particularly, when heat of an equipment supported by the structure must be evacuated by this structure, a known solution consists in charging, during the production of the part, the organic matrix with metal particles to improve the thermal conduction of the obtained material.
This solution presents however the defects of not to be realizable with all the methods for producing the composite material parts and to require a rather high load in proportion of the resin and fibers to ensure a significant increase of thermal conduction which induces a weight increase of the carried out part and a reduction in its mechanical resistance which must be taken into account at the design.